The present invention relates to improvements in screening devices for screening papermaking pulp fibers, and more particularly to an improved vertical centrifugal screen for pulp stock.
In the screening of pulp fibers, it is desirable to separate the fibers into their individual discrete form. The stock as fed to screens normally contains varying amounts of splinters and fiber bundles, and it is the primarly purpose of the screening operation to remove such undesirable coarse material so that a uniformly clean stock is available for the papermaking machine. While the problem of obtaining clean stock exists for all types of fibers, it is especially troublesome with groundwood fibers. Other types of screening devices include horizontal and vertical screens, although the present invention pertains particularly to a vertical centrigual screen wherein the stock is fed to the center of a screen, and the accepts, or acceptable fibers pass through openings in the screen to be collected in a housing surrounding the screen, and the rejects including splinters, fiber bundles, sheaves, particles of dirt and bark, are retained within the screen and pass out through a reject opening in the housing.
In vertical screens of the type to which the present improvements apply, it is important that continuous operation be achieved without plugging or impeding the continual screening flow of stock through the screen. It is also important to the device that a regulatable control of flow be achieved with structure of uncomplicated design and relatively easy to control. Centrifugal screens of the type described herein are usable in other industries, but the present invention finds particular advantage in the papermaking industry and will be described in connection with the screening of paper pulp. The present invention presents an improvement over screens heretofore used in providing a lower cost per unit of volume processed and requiring less space for installation. It also provides a structure which is more convenient for operation, particularly with respect to adjustment of the jet opening which controls the rate of flow of stock through the screening mechanism.
It is accordingly an object of the present invention to provide an improved structure for a centrifugal screen for screening paper pulp.
A further object of the invention is to provide an improved screen which will increase the screening flow through and will provide an improved resultant product, and which achieves an improved centrifugal effect without having to drive the screen or other parts in rotation.
In accordance with the principles of the present invention, a circumferential slice opening is provided which delivers a continuous radial jet at one end of the cylindrical screen and the radial velocity of the jet can be controlled by varying the pressure in the inlet chamber. The volume of flow can be controlled by varying the slice opening. The jet which emerges radially from the slice is then redirected downwardly and at the same time given a slight tangential component of velocity by unique deflector vanes. This results in a spiral flow of the liquid along the inner surface of the cylindrical screen. The centrifugal force which is determined by combined effects of jet velocity, spiral angle and basket diameter, cause dewatering or screening of the stock to occur. It is unnecessary to encounter the expense of a construction which provides for supports and power drive in rotation. Depending on the size of the openings in the cylindrical screen, particles whose size exceed a certain limit will be retained and will eventually fall off the screen surface to be collected in a coarse fraction compartment. Most of the liquid and small particles which pass through the screen are collected in a fine fraction compartment. The design of the structure is such that it accomplishes a low manufacturing cost, and the structure provides that the piping which is supported by collection compartments provides the mechanical support for the inlet chamber. Machining including turning and boring is required on a minimum number of components which comprise the head assembly, and the remainder of the machine includes standard pipe fittings and simple stainless steel fabrications.
Other objects, advantages and features will be become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiment thereof in the specification, claims and drawings, in which: